Current status and challenges for unified understanding of bonding mechanism in solid particle deposition process

Cold spray (CS) technology is a "solid-phase particle deposition process without melting"; however, it has been established as an additive manufacturing technology that can be applied beyond the framework of one field of thermal spraying. The scope of application of this technology has expanded to include ceramics and polymers. There are other solid particle deposition processes besides CS, such as aerosol deposition (AD), which differ in the material type, size, impact speed, and temperature of the target particles. We can expect that there is a common intrinsic mechanism through which solid-phase particles are joined and deposited in the solid phase. This review summarizes previous studies on the mechanism of cold-spray deposition and bonding, which can be understood as a mechanochemical phenomenon in part, and it is driven by the deformation of the particles and the resulting change in the chemical state of the particle surface, and stabilization by contact in a short time. When we understand these issues correctly, the optimal mechanical conditions (material size and collision conditions) for joining particles of various materials will be systematically understood, and it will be possible to perform different fabrication processes from thin films to additive manufacturing without melting various materials.